Underplayed: I'd say Gragas, but I play with someone who almost always picks him, so Janna.
Underpowered: Evelynn or Veigar.
Overplayed: Same as underpowered, given how flawed these two champions are.
Overpowered: Don't think anybody's overpowered, as there isn't a champion that doesn't have a readily available counter.

Underplayed: Gragas/Janna, but I think this depends on where your ELO is. I don't see much of Rammus, so I may be low... :/
Underpowered: Eve/Janna
Overplayed: shen, ashe
Overpowered: Jax(for now), Shaco